At present, soft drinks such as juices, or alcoholic beverages such as beer are sold in a form that the drink is filled in containers such as glass bottles or PET (polyethylene terephthalate) bottles. When the drinks are on sale, in order to differentiate the product from other products and to improve its visibility for enhancing its commercial value, a heat-shrinkable label where printing is given thereon is provided around the outer surface of each container. Conventional examples of the material for the heat-shrinkable label include: polyvinyl chloride (PVC), polystyrene, and aromatic polyester.
On the other hand, recently, effective use of depletable resources becomes important and utilization of recyclable resources is the major issue. Now, the most attractive solution is the use of plant-derived plastics. The plant-derived plastics not only employ non-depletable resources as the base material and enable to save consumption of depletable resources at a time of plastic manufacturing, but also exhibit excellent recyclability.
Among the plant-derived plastic, particularly, polylactic acid resin attracts attention as an alternative for polystyrene and aromatic polyester (polyethylene terephthalate) in the field of film packaging material and injection molding, because polylactic acid resin is made of a lactic acid, as a base material, obtained by fermentation of starch, it can be mass productive in view of chemical engineering and exhibits excellent transparency and stiffness.
However, when polylactic acid is used as a base material of the heat-shrinkable label, although the label shows stiffness at room temperature, low-temperature shrinkage, and favorable natural shrinkage, the obtained label is extremely brittle and tends to cause uneven shrinkage and wrinkles at a time of thermal shrinkage, which is problematic. In addition, when the heat-shrinkable film made of polylactic acid is heated, crystallization is developed; thereby sufficient heat-shrinkable property cannot be obtained.
As a means to solve the above problems, a film made of polylactic acid resin in which copolymer ratio of L-lactic acid and D-lactic acid is adjusted is known (See Japanese Patent Application Laid-Open (JP-A) No. 2003-119367.). However, although the film can inhibit crystallization at a time of heating, problems of causing uneven shrinkage, wrinkles, and pocks by sudden shrinkage has not yet been solved, sufficiently.
Moreover, improvement of shrink finishing quality has been attempted by adjusting degree of crystallization of polylactic acid resin, blending aliphatic polyester resin, and so on (See JP-A No. 2001-011214.). However, compared with PVC series heat-shrinkable film, it cannot be said that the shrink finishing quality is sufficient.
Further, about polylactic acid resin, due to the brittleness of the material itself, when the polylactic acid resin is used solely to form a sheet, a film, or the like, sufficient strength cannot be obtained; hence it is difficult to use practically.
To the above problems, there are known methods by containing an aliphatic polyester other than polylactic acid (See JP-A No. 09-169896.), a polycaprolactone (See JP-A No. 08-300481.), a copolymer polyolefin such as ethylene-vinyl acetate copolymer (See JP-A No. 09-151310.), and so on. These methods are provided for the purpose of mainly improving brittleness of polylactic acid resin film while keeping its transparency; therefore, shrink finishing quality is still insufficient.
Still further, as means to improve brittleness of the polylactic acid resin, there are disclosed methods using: a film using polylactic acid and a polyolefin compound (See JP-A No. 2005-068232.); a molded product (See JP-A No. 09-316310.) and a composition (See JP-A No. 05-179110.) made of polylactic acid and a modified olefin compound; a molded product made of a polylactic acid and a syndiotactic polypropylene (See JP-A No. 10-251498.); a plasticized polylactic acid film made of: a polymer mainly containing lactic acid, an aliphatic carboxylic acid, and a plasticizer of an aliphatic polyester having liner molecule diol as the main component (See JP-A No. 2000-191895.); a biodegradability resin composition made of polylactic acid and an epoxidized diene block copolymer (See JP-A No. 2000-219803.); a lacitic acid polymer composition made of polylactic acid, an aliphatic polyester, and polycaprolactone (See JP-A No. 2001-031853.); and a polylactic acid resin composition made of: a crystalline polylactic acid and at least one rubber component selected from natural rubber and a polyisoprene (See JP-A No. 2003-183488.).
When the above-described polycaprolactone, modified olefin compound, epoxidized diene block copolymer, natural rubber, polyisoprene, and the like are mixed with lactic acid resin, improvement of impact resistance can be observed. Nevertheless, as the trade-off, transparency is significantly deteriorated; therefore these arts are not sufficient for the use which requires checking the content of the packaging or the like.
In addition, a method to improve impact resistance by mixing polylactic acid resin with an impact-resistance improver for e.g. a multi-layer structure of which shell-layer includes: polyacetal resin and diene rubber, natural rubber, silicone rubber, polyurethane rubber, or methyl (meth)acrylate; and of which core-layer includes: at least one selected from styrene unit and butadiene unit (See JP-A No. 2003-286400.). However, the obtained heat-shrinkable film is not sufficient.
Further, a method by blending polylactic acid resin with a grafted copolymer obtained by graft polymerization of gum polymer and vinyl-monomer (See JP-A No. 2004-285258.) is proposed; the obtained heat-shrinkable film is not sufficient.
While, methods by mixing a polylactic acid resin and an acrylic base material (See JP-A No. 2004-269720 and JP-A No. 2005-171204.) are proposed; the methods are provided for the purpose of improving thermal resistance and transparency; therefore, it is difficult for the method to be adopted to improve rupture resistance. Further, an oriented film, in which a polylactic acid resin having a particular mass average molecular mass and a polymethacrylic acid ester resin are mixed, is known (See JP-A No. 2005-036054.). However, the art disclosed in Patent document 18 is provided for the purpose of mainly improving thermal resistance and transparency of polylactic acid resin film; hence, it is difficult for the method to be adopted to improve rupture resistance of the heat-shrinkable film of the present invention.